Traditionally, VCSEL dies are vertically mounted to a printed circuit board, or PCB, with light emitting from the same surface as the electrical contacts. The PCB is usually made of FR4 or ceramic. Other mounting substrates could include metals such as invar or plastics housings such as LCP. As shown in the prior art of FIG. 1, a TO can assembly 12 has wire bonds 16 used in electrically connecting the VCSEL die 14. Wire bonds 16 are more susceptible to damage than solder bumps, and are generally avoided if possible. In addition, wire bonding is inconsistent in terms of variance in electrical properties. As the wire lengths tend to vary, variance exists in resistance, inductance, or capacitance of the lines. As shown in FIG. 1, the TO can""s base comprises a header 20 and a conductive spacer 18. A metallic structure 22, referred to as a can, provides a hermetic seal for a VCSEL laser array 14. Optical signals 26 exit the TO can 22 through a lens 24, and may be appropriately coupled into a waveguide (not shown). A method of attaching the VCSEL die using metal to metal contacts on the pads such as solder bumps or stud bumps can make closer connections that are more consistent in electrical variance and offer greater structural stability than wire bonds. This method of attaching is commonly referred to as flip chipping. Wire bonding adds to the overall height in the package more so than flip chipping, as shown in FIG. 1. In addition, flip chipping allows for a waveguide and/or lens structure to be placed closer to surface of light emission. As a result, the coupling efficiency between the active optical device and waveguide/optical fiber could increase.
As stated above, a VCSEL laser die often contains electrical contacts on the same surface of light emission. Flip chipping a laser die to a substrate can eliminate the need for complicated lensing devices necessary to capture enough light. Because flip chipping can eliminate use of wire bonds on an active optical surface, an optical fiber or waveguide can be closer to the optical port. If the distance from a coupling device to the optical port decreases, more divergent light can be collected before being obstructed by covering features or interfering with adjacent optical devices. This in turn may preserve signal integrity.
Flip chipping of IC""s is widely understood. Yet, the flip chipping of VCSEL or photodiode dies is a newer practice with room for modifications and improvements. Typically, conventional stud bumps or solder bumps establish electrical connections between conductive traces and optical devices. A solder bump can structurally attach the optical device to a substrate or similar device, but a stud bump is typically used in conjunction with an adhesive (the structural member). Adhesive selection becomes important under large temperature variations. Given an assembly going through a tin-lead solder reflow oven, bonded surfaces could shift in relation to each other, and the adhesive, or solder bump, must hold the positions of the devices in relation to each other.
If an adhesive is placed on the sides of the die and not on the optical device""s surface attached to the substrate, a number of problems could arise if an air-gap remains between the substrate and the optical device. Foreign materials could possibly find their way into the open space surrounded by the solder, contaminate the optical ports, and interfere with signal integrity. During aqueous washing of the assembly, unwanted chemicals may enter the region of the optical array, contaminating the optical array and depreciating signal integrity. For this reason, an adhesive is better suited between the two surfaces of contact. Yet, if attaching a VCSEL die to a substrate, where the optical emission surface of the optical array is attached to the substrate, the adhesive must allow optical signals to pass through.
As flip chipping an optical device to a substrate can enable closer proximity of a waveguide to the port, this can enable coupling of more divergent optical radiation, thus increasing the total amount of light gathered and eliminate the need for lens mechanisms. As it is desirable to uniformly collect light over the optical source""s total angular emission field, it is not necessarily advantageous to gather as much light as possible. Capturing too much light through an optical fiber or waveguide could cause a few problems, one of which is eye safety. As a laser can cause permanent damage to the human eye, it is imperative to ensure that a laser""s output does not come in contact with a human eye in a hazardous manner.
Another possible consequence in gathering too much light involves the inability of a receiving optical device to process the light energy. A photodetector may provide an electrical output that is proportional to the amount of light energy from a transmitting device. If the input signal to a photodetector contains too much light energy, the photodetector could become saturated. That is, the linear proportionality between the incoming light energy and the outgoing electrical signal could diminish, and the photodetector may not respond accordingly beyond a certain range of light energy. Additionally, if the photodetector has not already saturated, a signal processor receiving the electrical signal from the photodetector could become saturated. That is, the signal processor""s limits will have been reached because the value of the input electrical signal could be too high. Because of these two consequences in gathering too much light energy, it is necessary to appropriately control an optical signal.
In this invention we provide a novel way to couple light from an optical device, into a waveguide, and subsequently into an optical fiber. The invention may simultaneously function as a waveguide, a structural member, a protective means for optical ports, and an optical attenuator. It may allow coupling of divergent light while transmitting an appropriate amount of optical energy to a receiving device. In addition, the invention may promote eye safety while maintaining signal integrity.
A method and apparatus are provided for providing an electro-optic interface for exchanging information signals. The method includes the steps of disposing an optical array adjacent a first side of an optically transparent substrate, such that a plurality of transmission paths of the optical array pass directly through the substrate, applying an optically transparent underfill between the substrate and adjacent optical array with the plurality of transmission paths of the optical array passing directly through the underfill and coupling a plurality of optical signals of the optical array through the optically transparent underfill and optically transparent substrate between the optical array and an optical connector.